Liquid electrolyte is applied in most commercial rechargeable lithium batteries. Alternatively, a so-called gel polymer, i.e. polymer with a very large fraction of liquid electrolyte is applied. These electrolytes have relatively high ionic conductivity, whereas the Li transference number thereof is typically below 0.5, i.e., tLi+<0.5. As a result, anionic diffusion dominates during fast charge and discharge.
This low Li transference number causes dramatic and undesired effects. More specifically, during fast charge or discharge, anions counter-diffuse and a gradient of salt concentration is established in the electrolyte, whereby the electrolyte kinetically depletes. Consequently, the electrolyte conductivity decreases to cause poor rate performance. Furthermore, the electronic potential of lithium plating is altered, and particularly during fast charge in a region near to the anode, the electrolyte may exceed the electronic stability window, causing accelerated reductive electrolyte decomposition.
As a result, it is strongly desired to slow down the anionic diffusion. In an ideal case, a Li-ion conductive membrane with the Li transference number, tLi+=1, separates the electrolyte-soaked anode and cathode; however, no practical ways that are able to achieve it have been found yet. Generally, the charge transfer of lithium at the solid electrolyte-liquid electrolyte interface of such membranes is too slow.
Numerous patents suggest composites of polymer (e.g., PEO) with inorganic fillers (e.g., nano-Al2O2 or silica) to create solid electrolytes with improved conductivity and an increased Li-transference number. However, in spite of the significant progress, the achieved transport properties are far away from real commercial requirements. It is doubtful if further progress can be achieved. The improvement in these composites is attributed to structural changes (less crystallinity) of the polymer near to the filler particle, and thus, further significant improvements are not likely.
Another approach is known in the area of solid electrolytes. Here metal-halogenite solid electrolyte ionic conductors like lithium iodite (LiI) or silver halogenides (AgCl, AgBr, AgI) tc.) are “heterogeneously doped” using submicrometer particles (e.g., Al2O3). In this approach, the transport properties can be improved because the grain boundary conduction exceeds the bulk conduction. The increase of grain boundary conductivity is explained by the concept of space charge. This concept has in detail been summarized in “Ionic conduction in space charge regions” (J. Maier, Prog. Solid State Chem, 23, 171).
A similar concept has been applied to liquid electrolytes. “Heterogeneous doping” of liquid electrolytes has been described in “Second phase effects on the conductivity of non-aqueous salt solutions: soggy sand electrolytes” (A. J. Bhattacharya and J Mair, Advanced Materials 2004, 16, 811) and “Improved Li-battery Electrolytes by heterogeneous Doping of Nonaqueous Li-salt solution” (A. J. Bhattacharya, Mockael Dolle and J Mair, Electroch. Sol. State Letters 7 (11) A432). In these cases, addition of fine particles such as Al2O3, TiO2, SiO2, etc. to the electrolyte results in “soggy sand electrolytes”. Soggy sand means that rigid solid particles (which may have small sizes) coexist with a liquid phase. Among them, in the case of SiO2, an improvement of transport properties is achieved; however, it is not recommended to apply SiO2 because in real batteries it causes undesired side reactions consuming lithium, which has been investigated and described in detail in chapter 6 of Zhaohui Chen's PhD thesis (Dalhousie university, Halifax, 2003).
Therefore, there is strong need for liquid electrolyte being able to allow the fast diffusion of lithium ions but that hinders a fast anionic diffusion.